There is disclosed herein a novel method for aligning liquid crystal molecules for use in liquid crystal displays.
Liquid crystal materials and liquid crystal displays are well known. In a publication by E. Merck of Darmstadt, West Germany, entitled "Licristal", the state of liquid crystal technology is discussed. The publication carries an identifying number of 21/II-23/491/3.5/372. Other references on liquid crystal materials are available, such as deGennes, Physics of Liquid Crystals, Oxford University Press, 1974, and S. Chandrasekar, Liquid Crystals, Cambridge University Press, 1978.
There are three basic types of liquid crystals, namely, smectic, nematic and cholesteric. The principal differences between the types of crystals are in the molecular arrangements.
As can be appreciated, the physical properties of the liquid crystals will vary between types. Furthermore, this variation may make one type more or less desirable for different applications.
Liquid crystal displays using various types of liquid crystals are at a fairly advanced state. For example, products such as LCD wristwatches and LCD calculators are commercially available. In order to make a display, the liquid crystal is usually held in a sealed envelope between two substrates, at least one of which is optically transparent. The substrates are usually glass.
The surface of the substrates which are to contact the liquid crystal are treated before the display is assembled in order to obtain the desired alignment of the liquid crystal molecules for the particular type of display and to maximize the contrast ratio.
Presently there are two principal types of surface treatments. They include (a) applying various materials on the surface with which the liquid crystal molecule will interact or (b) mechanically abrading the surface so that the molecule interacts with the surface formation. For example, lecithin may be deposited in order to obtain homeotropic alignment or organics may be deposited by rubbing with a tissue to obtain homogenous alignment.
The other technique is to physically treat the surface to form microgrooves or striations. This can be done by abrading or cutting, rubbing with a diamond paste, or vacuum depositing siicon monoxide using the Jannings technique. See Priestly, et al., Introduction to Liquid Crystals, Plenim Pub. Corp., New York; Ch. 12, "Liquid-Crystal Displays--Packaging and Surface Treatments", L. A. Goodman, 1976; and A. R. Kmetz and F. K. von Willisen, Nonemissive Electro-Optical Displays, Plenim Pub. Corp., 1976, New York.
Using these techniques, homogenous and twisted structures can be obtained with nematic liquid crystals. In the homogenous structure the substrates are arranged so that when the cell is assembled, the striations and the molecules are parallel and aligned in a single direction. In other words, the substrate striations are parallel to one another and the molecules of the liquid crystals therebetween are macroscopically speaking, or on an average, parallel in the same direction.
In the twisted structure, the liquid crystal display is assembled with the striations on each of the two substrates being aligned in the same direction. However, one substrates may then be rotated, by usually less than 90.degree. (perhaps between 80.degree. and 90.degree.) relative to the other substrate, thereby causing the liquid crystal molecules to assume a helical shape or twisted nematic.
Twisted nematics may also be formed by initially placing the substrates so that the striations are transverse to each other. Then when the cell is filled, the liquid crystal molecules will assume the helical configuration. This process eliminates the need to rotate the substrates in order to obtain the alignment.
In the twisted nematic, the molecules are angularly displaced relative to each other in such a manner that the angular displacement varies continuously from the surface of one substrate to the surface of the other substrate. This is the twisted nematic widely used in LCD displays of many kinds, such as watches, calculators, multiplexed displays, alpha-numerics, etc.
It has been determined that the existing methods for treating the substrates each has drawbacks relating to cost and ease of handling. For example, rubbing with a tissue may leave an undesirable residue and may cause chipping at the edges of the substrates which result in undesirable scoring of the substrate; diamond paste cutting may be expensive and leave an undesirable residue that may interact with the liquid crystal molecule. The process of vacuum depositing a silicon oxide (SiO) surface film has the disadvantage of requiring large, elaborate and expensive equipment as well as a long set-up time.
It is therefore the object of this invention to provide an improved technique for surface treating the substrates used in liquid crystal displays or cells for assuring directional alignment of the liquid crystal molecules.
These and other objects of this invention will become apparent from the following description and the appended claims.